1. Field of the Invention
The present invention relates to a braking force control device, disposed in a vehicle, for controlling a braking force in response to operation of a brake pedal.
2. Description of the Related Art
There have conventionally been known braking force control devices that create, on the basis of signals from a wheel speed sensor, speed signals approximating vehicle acceleration or vehicle speed to control braking from a comparison of these and carry out anti-lock braking control (ABS control) to prevent wheel lock. Further, as disclosed in Japanese Patent Application Laid-Open (JP-A) No. 6-179361, for example, when operational speed and the like of a brake pedal exceeds a predetermined threshold, it is judged that rapid braking is required, whereby so-called brake assist control (BA control) is performed to increase braking force with respect to the force at which the brake pedal is depressed.
In order to appropriately operate BA control under various conditions, JP-A No. 9-263233 discloses a braking control device in which initiation criteria of BA control are altered in accordance with brake pedal operation amount, position at which the brake pedal is depressed, stroke, oil pressure of a master cylinder, depression force, depression speed and the like. Further, in JP-A No. 10-273022, there is disclosed a braking control device in which, in order to improve vehicle stability when BA control is operated at the time a vehicle turns, a threshold for initiating BA control is lowered when the turning condition is a condition in which the vehicle is stable and BA control for the rear wheels is not permitted when the vehicle is outside the region of stability.
In a braking force control device that performs ABS control and BA control, BA control is performed on the basis of depressing the brake pedal and the like, and ABS control is performed in response to braking conditions.
However, depending on road surface conditions, sometimes braking initial wheel slip becomes larger due to an increase in oil pressure when BA control is conducted, and the behavior of the vehicle becomes unstable when ABS control is initiated on the basis of the wheel slip.
The present invention has been devised in view of the above facts. It is an object of the present invention to provide a braking force control device for preventing vehicle behavior from becoming unstable when brake assist control and anti-lock braking control are performed.
In order to accomplish this object, the present invention is a braking force control device comprising: wheel speed detecting means for detecting wheel speed of a wheel; road surface xcexc slope estimating means for estimating a slope of the braking force with respect to slip speed of the wheel on the basis of the wheel speed detected by the wheel speed detection means; braking operation detecting means for detecting a braking operation by which a brake pedal is depressed; and assist control means for assisting braking of the wheels by the braking means for braking the wheels by a braking force generated in response to the braking operation by which the brake pedal is depressed, on the basis of the estimation results of the road surface xcexc slope estimating means and braking operation conditions detected by the braking operation detecting means.
According to the present invention, assist of braking is conducted in response to braking operation conditions when the brake pedal is depressed. Namely, braking of the wheels is assisted.
Further, assist of braking is conducted on the basis of the road surface xcexc slope that the road surface xcexc slope estimating means estimates by calculating and the like on the basis of wheel speed detected by the wheel speed detecting means.
Accordingly, appropriate brake assist in response to road surface conditions becomes possible, and it becomes possible to prevent vehicle behavior from becoming unstable and steerability from dropping even if anti-lock brake control is conducted.
In the present invention, it is preferable that the braking operation conditions include a determining means to determine whether or not assist conditions set on the basis of the estimation results of the road surface xcexc slope estimating means have been exceeded, and the assist control means assists braking on the basis of determination results of the determining means. In this case, it is further preferable that the assist conditions are lowered when the road surface xcexc slope estimated by the road surface xcexc slope estimating means is high, and the assist conditions are raised when the road surface xcexc slope estimated by the road surface xcexc slope estimating means is low.
Further, in the present invention, it is preferable that the device further includes assist amount setting means for setting an assist amount at the time braking of the wheels is assisted on the basis of estimation results of the road surface xcexc estimating means, and that the assist control means assists braking at an assist amount set by the assist amount setting means. In this case, it is further preferable that the assist amount is made large when the road surface xcexc slope estimated by the road surface xcexc slope estimation means is high, and that the assist amount is made small when the road surface xcexc slope estimated by the road surface xcexc slope estimating means is low.
Moreover, in the present invention, it is preferable that the assist amount setting means corrects the assist force, while braking of the wheels is assisted, on the basis of estimation results of the road surface xcexc slope estimating means in order for assist of the braking in response to changes in road surface conditions to become possible.
Still further, in the present invention, it is preferable to set assist conditions of rear wheels to be lower than assist conditions of front wheels and to set an assist amount of rear wheels to be smaller than an assist amount of front wheels, so that braking assist of the rear wheels is suppressed even more than braking assist of the front wheels, whereby stability and steerability of the vehicle are ensured.
In addition, it is preferable that it is possible to assist only braking of the front wheels when an estimation result of the road surface xcexc slope estimating means is low.
According to the present invention described above, because brake assist is conducted in response to road surface conditions, excellent effects can be obtained in that brake assist can be prevented from being conducted unnecessarily, and it is possible to ensure stability and steerability of the vehicle while the wheels are appropriately braked.